The present disclosure relates generally to an imaging module and a system for, and a method of, electro-optically reading a target by image capture, and more particularly, to illuminating the target to be read by alternately energizing and deenergizing light sources in a timed manner that mitigates specular reflection from interfering with the reading and at a blink rate that minimizes unpleasant blinking light patterns from visually annoying operators and people in proximity with the module/system.
Solid-state imaging systems or imaging readers have been used, in both handheld and/or hands-free modes of operation, to electro-optically read targets, such as one- and two-dimensional bar code symbol targets, and/or non-symbol targets, by image capture. A handheld imaging reader includes a housing having a light-transmissive window and a handle held by an operator. An imaging module, also known as a scan engine, is supported by the housing and aimed by the operator at a target during reading. The imaging module includes a solid-state imager or imaging sensor with an array of photocells or light sensors, which correspond to image elements or pixels in an imaging field of view of the imager, and an imaging lens assembly for capturing return light scattered and/or reflected from the target through the window, and for projecting the return light onto the array to initiate capture of an image of the target. Such an imager may include a one- or two-dimensional charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) device, with global or rolling exposure shutters, and associated circuits for producing and processing electronic signals corresponding to a one- or two-dimensional array of pixel data over the imaging field of view. A typical imager may operate at a frame rate of about 30-60 frames per second (or Hz) to acquire an entire target image. Each frame is a unique, consecutive image acquired by the imager.
In order to increase the amount of the return light captured by the array, especially in dimly lit environments and/or at far range reading, the imaging module generally also includes an illuminating light assembly for illuminating the target with illumination light for reflection and scattering therefrom. The illumination light is typically emitted from illumination light sources, e.g., light emitting diodes (LEDs), and illumination lenses may be used to modify the illumination light over an illumination field. Although generally satisfactory for its intended purposes, the use of the LEDs has sometimes proven to be disadvantageous, because a portion of the emitted illumination light incident on the target is reflected therefrom back into the imaging field of view of the image sensor. The reflected portion of the illumination light may sometimes create undesirable bright or “hot” spots in the imaging field of view, and these hot spots, also known as glare, constitute specular light, which can overload, saturate, obscure, interfere, and sometimes even “blind” the image sensor, especially on an initial reading attempt, thereby degrading reading performance. The effects of specular light may be worsened when the target is printed on a label having a glossy surface, or when the target itself has a highly reflective, outer surface, because such glossy, reflective surfaces promote mirror-like, specular reflections.
To mitigate such specular reflections, it is known to energize only a first LED throughout a first frame of the imager to capture a first image of the target, and to subsequently energize only a second LED throughout a second frame of the imager to capture a second image of the target. A main controller can then combine or stitch an area of the first image that is substantially free of specular reflection with an area of the second image that is also substantially free of specular reflection to form a composite image, and then process the composite image to read the target without interference from specular reflections.
Although generally satisfactory for its intended purposes, this approach cuts the effective frame rate in half since it takes two frames to produce the composite image. A slow frame rate is not desirable, especially in those applications where aggressive, fast reading performance is needed. In addition, each LED is energized, pulsed, or blinked, once every two frames. Put another way, each LED has a blink rate that is half of the frame rate. If the imager operates at a frame rate of about 30-60 Hz, then the blink rate is 15-30 Hz. Bright illumination pulses emitted by the LEDs shining out of the window, especially at pulse rates below 50-60 Hz, can be annoying or uncomfortable to the operator, or to a consumer or others standing nearby the reader.
Accordingly, there is a need to increase the blink rate to a comfortable level while mitigating specular reflections from interfering with the reading of the target.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The module, system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein.